1. Field of the Invention
The present invention relates to a magnetic biasing apparatus for generating a magnetic field required to record or erase information signals on or from a magneto-optic recording medium.
2. Description of the Prior Art
In recent years, a magneto-optical disk has come to be recognized as a sophisticated high-density recording medium and has rapidly been popularized on the market. In addition, there is growing interest in a magnetic field modulated magneto-optical recording method whereby immediate writing (overwriting) is possible. In this method, information recording is performed primarily by modulating a magnetic field, and a magnetic biasing apparatus plays a very important role in magnetic field modulation.
The magnetic field modulation for the magneto-optical disk is discussed in detail hereinafter.
The magneto-optical disk comprises a transparent disk substrate having a number of generally concentric pre-grooves defined therein, a plurality of thin films laminated one upon another on one surface of the substrate, and a protective coating overlying the thin films. The thin films are generally a protective film, a recording film, and a reflection film.
In recording information on the magneto-optical disk, the magneto-optical disk is rotated about the center thereof, and a laser beam throttled by an objective lens is applied thereto from the substrate side and is focused on a desired portion of the pre-grooves by the use of a focusing control means. Thereafter, such desired portion is scanned along the pre-grooves by a tracking control means.
In reading the information recorded on the magneto-optical disk, a magneto-optical effect is utilized. More specifically, when a polarized laser beam is applied to the recording film of the magneto-optical disk, reflected light thereof causes a rotation of a polarized light plane in compliance with the direction of magnetization of the recording film. The angle of rotation of this polarized light plane is then converted into an electric signal by the use of an optical element, for example a polarizer, a photodetector, and the like in order to read the information.
The recording utilizes a non-modulated laser beam and a modulated magnetic field. The modulated magnetic field is generated by a magnetic biasing apparatus having a magnetic head placed on the side of the protective coating of the magneto-optical disk. The magnetic head comprises a core made of a high-permeability material and a coil wound therearound, and generates a modulated magnetic field by controlling an electric current which is allowed to flow in the coil.
Because the magneto-optical disk to which the laser beam has been applied is partially heated, the power of the laser beam is controlled so that the temperature of the recording film at that portion of the magneto-optical disk to which the laser beam has been applied (this portion is hereinafter referred to as a recording spot) may become a temperature in the neighborhood of the Curie point of a magnetic film constituting the recording film. The magnetic film has a perpendicular magnetic anisotropy and loses magnetization at temperatures over the Curie point. When the temperature of the magnetic film is reduced below the Curie point, the magnetization appears, and the direction thereof depends upon the direction of the magnetic field applied from outside. By making use of this property, the recording is generally performed by controlling the direction of magnetization of the magnetic film.
In the recording film of magneto-optical disks currently in use, the magnitude of the magnetic field which must be applied from outside during recording is in the range of .+-.100 to .+-.400 Oe. In order to apply this magnitude of the magnetic field to the recording spot at a modulated frequency over several hundreds kHz, the inductance of the magnetic head is required to be below several .mu.H. To this end, it is necessary to reduce the size of the core of the magnetic head and to make the distance between an open end of a magnetic circuit of the magnetic head and the recording spot of the recording film as small as possible.
In order to bring the open end of the magnetic circuit of the magnetic biasing apparatus near the recording spot on the magneto-optical disk, a sliding method is disclosed in, for example, Japanese Laid-open Patent Publication No. 63-58605, and a floating method is disclosed in, for example, Japanese Laid-open Patent Publication No. 63-229643.
The sliding method is a method wherein the open end of the magnetic circuit of the magnetic head is opposed to the protective coating of the magneto-optical disk while the slide member mounted on the former is in direct contact with the latter.
The floating method is a method wherein the protective coating of the magneto-optical disk is opposed to and spaced away from the open end of the magnetic circuit of the magnetic head with a very small empty space defined therebetween. The floating method includes two types, in one of which the magnetic head is integrally formed with or otherwise secured to a generally flat floating member, and air is introduced between the protective coating and the floating member by making use of the relative speed between the floating member and the protective coating of the magneto-optical disk, thereby maintaining the empty space. In the other type of floating method, the empty space is maintained very small by a servo system which comprises a position detection means, a position control means, and the like.
The sliding method includes a problem in connection with sliding durability. More specifically, the problem is encountered as to which material should be chosen as the material interposed between the protective coating and the magnetic head. According to the Japanese Laid-open Patent Publication No. 63-58605, the slide member is coated with, for example, a synthetic resin or covered with a thin film. Alternatively, the slide member is made of fluorocarbon resin, boron nitride, graphite, or a composite material based on these materials. Furthermore, this publication teaches that the slide member may contain a lubricant such as, for example, a fatty acid, a higher alcohol, a fatty acid ester or the like.
Based on the contents of this publication, inventors of the instant application prepared a slide member and incorporated it into a magnetic head of a magnetic biasing apparatus. Using this magnetic biasing apparatus, information recording was performed on a magneto-optical disk while a sliding condition is maintained for a long time. As a result, it was discovered that read signals were deteriorated in quality.
The inventors investigated causes of such deterioration in detail and have discovered that shortage of a recording magnetic field or a variation in the interval between recording pits deteriorated the signal quality. The shortage of the recording magnetic field was mainly caused by the fact that the sliding operation brought about a flow and deformation of the surface of the slide member, and the recording film of the magneto-optical disk was moved slightly away from the open end of the magnetic circuit of the magnetic head. As a result, a predetermined magnitude of external magnetic field was not applied to the recording film. The variation in the interval between the recording pits was mainly caused by the fact that the sliding operation brought about abrasion of the surface of the slide member and changed the surface state. Because of this, the friction force varied according to the position on the protective coating, which in turn caused a variation in speed of the magneto-optical disk. As a result, the variation in the interval between the recording pits occurred.
In the air-floating method, if the relative speed between the protective coating and the floating member is reduced, the floating stability is lost, and the distance between the magnetic head and the recording film varies. This causes a variation in magnitude of the applied magnetic field and deteriorates the quality of the recording signals.
In the servo-floating method, the position detection means and the position control means require relatively expensive parts, and each of them has a complicated structure and requires a relatively large installation space. Such means does not meet requirements of users towards an inexpensive and thin-structured apparatus.